An in situ flow tube system for direct measurement of N₂O₅ heterogeneous uptake coefficients in polluted environments

<p>The heterogeneous reactivity of dinitrogen pentoxide (N₂O₅) on ambient aerosols plays a key role in the atmospheric fate of NO_<i>x</i> and formation of secondary pollutants. To better understand the reactive uptake of N₂O₅ on complex ambient aerosols, an in situ experimental approach to direct measurement of N₂O₅ uptake coefficient (<i>γ</i>N₂O₅) was developed for application in environments with high, variable ambient precursors. The method utilizes an aerosol flow tube reactor coupled with an iterative chemical box model to derive <i>γ</i>N₂O₅ from the depletion of synthetically generated N₂O₅ when mixed with ambient aerosols. Laboratory tests and model simulations were performed to characterize the system and the factors affecting <i>γ</i>N₂O₅, including mean residence time, wall loss variability with relative humidity (RH), and N₂O₅ formation and titration with high levels of NO, NO_<i>x</i>, and O₃. The overall uncertainty was estimated to be 37&thinsp;%–40&thinsp;% at <i>γ</i>N₂O₅ of 0.03 for RH varying from 20&thinsp;% to 70&thinsp;%. The results indicate that this flow tube coupled with the iterative model method could be buffered to NO concentrations below 8&thinsp;ppbv and against air mass fluctuations switching between aerosol and non-aerosol modes. The system was then deployed in the field to test its applicability under conditions of high ambient NO₂ and O₃ and fresh NO emission. The results demonstrate that the iterative model improved the accuracy of <i>γ</i>N₂O₅ calculations in polluted environments and thus support the further field deployment of the system to study the impacts of heterogeneous N₂O₅ reactivity on photochemistry and aerosol formation.</p>